An optical pickup device is used for recording and reproducing optical disks such as CD, DVD, and Blu-ray (registered trademark) disk. In the optical pickup device, an output light from a light emitting element such as a laser diode is guided to an objective lens through various lenses, prisms, mirrors, and the like, and the light is concentrated on an optical disk. Then, a return light from the optical disk is received by a photodiode through various lenses, mirrors, and the like. Thus, an optical signal is converted to an electrical signal by the photodiode.
In this configuration, the optical component such as various lenses is placed and fixed at a predetermined position on an optical path of a pickup case. Fixing the optical component requires a high accuracy of the order of sub-micron. Most commonly, the optical component is positioned with a jig, and an ultraviolet curable adhesive is applied to a predetermined position to fix the optical component by irradiating ultraviolet light. However, the optical component fixed with the ultraviolet curable adhesive is not an ideal configuration because of fluctuations in application positions and amounts of the adhesive. A long-term displacement of the optical component is likely to occur, resulting in a decrease in reliability of the optical pickup device. Further, the annealing time and the ultraviolet irradiation time should be increased in order to stabilize and fully cure the adhesive. This has created a problem of productivity.
Thus, to replace the fixing method using an adhesive there has been proposed a method of welding an optical component to a case by a laser beam, in order to increase the positional stability of the optical component and its productivity. This laser welding technique is used not only for fixing optical components, but also for fixing various types of components in industrial areas. In laser welding, in general, welding is performed on a line or circle by scanning a laser source or a fixing jig, in order to ensure the welding area.
However, when the laser source or fixing jig is scanned, excessive welding is likely to occur in particular at the scan end portion, often resulting in a poor state of welding such as generation of void. Cycloolefin resin is the most commonly used lens material for the optical pickup, and PPS (polyphenylene sulfide) is the resin often used for the pickup case. Particularly, the PPS resin contains glass filler to increase rigidity and has a tendency to have a low linear expansion coefficient.
Thus, when the laser-welded or heated resin is rapidly cooled, considerable stress occurs as a function of the difference in the linear expansion coefficient between the lens material and the pickup case material. Particularly large residual stress occurs at the end portion of scan welding. As a result, peeling often occurs at the end portion in the welding process, which has been a problem to be solved. In addition, the reliability test has confirmed that the peeling necessarily develops from the end portion. Thus, in order to take advantage of the unique characteristics of laser welding, such as an increase in the positional stability of the optical component as well as a short tact time line, it is necessary to have a structure that prevents the peeling of the end portion in laser scanning.
JP-A No. 20867/2004 describes that in a film unit with lens and made from plural parts, the production cost is reduced by welding a plastic spherical lens to a cover by a laser beam, without using an adhesive or a special component.
JP-A No. 261986/2004 describes that the thickness of a laser transmitting resin is continuously changed in an overlapping corner portion, in order to prevent occurrence of a malfunction, such as poor welding caused by the difference in the movement of the laser beam between the outer periphery and the inner periphery.
JP-A No. 354438/2004 describes that an optical component of resin is joined to a resin holder, by evaporating the resin of the resin holder by laser with an inert gas discharged, and by depositing the melted resin onto the optical component. This enables fixing without aberration.
JP-A No. 119050/2005 describes that a convex portion is provided on a surface of a laser transmitting resin to change the optical path length. This eliminates the nonuniformity of energy distribution in a melting spot at relatively low cost.
JP-A No. 231308/2005 describes that defects such as voids are prevented from occurring in the welding line between a transmitting resin and an absorption resin, by diverting the laser beam from the welding line, and by stopping irradiation of the laser beam at the end portion thereof to cause defects to occur only at the end portion.
With the technique disclosed in JP-A No. 20867/2004, it is possible to fix the lens with high accuracy. However, when the lens is joined at a point, it is difficult to ensure a large welding area to provide the necessary welding strength. On the other hand, when welding is performed by scanning a laser beam, nonuniformity is likely to occur in the usual manner and result in poor welding at the end portion of laser scanning. However, in JP-A No. 20867/2004, there is no description of the above problem and its solution.
With the techniques disclosed in JP-A Nos. 261986/2004 and 119050/2005, it is possible to change the shape of the laser intensity distribution by varying the thickness and shape on the transmitting resin side. This structure is suitable for large parts such as automobile components. However, the optical component of the optical pickup is very small of millimeter order. Thus, even if the thickness of the laser irradiation part is somewhat changed on the transmitting resin side, there is a small change in the laser intensity distribution as a function of the optical path difference.
With the technique described in JP-A No. 354438/2004, it is difficult to deposit the melted resin at a certain amount and at an appropriate position when the inert gas is discharged. Thus the yield is low. Further, joint irregularities are likely to occur. As a result, it is difficult to ensure good welding performance.
With the technique described in JP-A No. 231308/2005, in particular, generation of void can only be prevented. However, there remains the problem of peeling due to excessive residual stress at the end portion.